The present invention relates to the field of quality control. More particularly, it relates to the inspection of industrial "scene", such as manufactured objects, for example, to determine automatically the presence or absence of proper parts, their identification, and their position or orientation, where color is a factor in the inspection process.
In any light image, there are three major components forming the composite light image. These components are luminance or intensity, hue, and saturation. In prior art electronic imaging inspection apparatus, monochrome video cameras have been used which respond only to the intensity or luminance component of the composite image. The monochrome video camera produces a signal, as noted, representing the intensity pattern of the object viewed. That signal is presented to an image processor which assigns a binary or "gray-scale" digital value to each pixel of the image in accordance with the luminance or intensity of the light signal at that pixel. The digital representation may then be compared with a prerecorded standard signal to provide an acceptance or rejection control of the examined object. On the other hand, in more sophisticated systems, the signal may be used by a programmed computer to compute the area, perimeter, coordinants, orientation, etc. of the parts. Additionally, the output signal may be applied to a video monitor for observation by a human observer.
The parts of a complex object are distinguished by the contrasts between them and surrounding elements or background. A difficulty arises when the parts differ from a background or from one another, only by color or color coded markings.
It will be remembered that the complete light image of an object has three characteristics, namely hue, saturation, and luminance. The hue and saturation components together are referred as the chroma or chrominance of the image. As noted, the output of the monochrome video camera contains only the luminance component. Accordingly, adjacent elements in an object which are of different colors and yet have the same luminance or brightness, may be indistinguishable, having the same shade of gray, in a monochrome presentation. Exemplary of such objects which would be indistinguishable on the basis of luminance alone, are found in the inspection of electronic component boards wherein resistors of various values differ from one another only in the color coding carried thereby. Similarly, resistors and capacitors may be distinguished from each other only in the color representation. Numerous other exmples may be found.
Heretofore, efforts have been made to overcome the aforementioned shortcoming of monochrome inspection techniques by using a color video camera, then processing the individual red, cyan, and blue signals provided thereby with three separate image processing devices to assign digital values for each of the color components. Thereafter, the results are combined and compared to determine the color as well as the other features of the part being observed. Such systems are quite costly in that they involve three separate image processing paths, and they take considerably more time in operation than is desirable.
In the above referenced U.S. Pat. No. 4,590,511 issued May 20, 1986, there is shown an inspection system which derives from the single, composite output signal of a color video camera, a signal which is interpretible by gray-scale image processing equipment. The system of the co-pending patent application not only distinguishes the presence and location of the parts of an object represented by their luminance, but also is capable of distinguishing the hue component of the image. In some instances, however, hue or luminance are not sufficient to distinguish objects; a measure of the saturation component is also needed.